There is a known way of using a piezoelectric actuator to control the opening and closing of an injector in an internal combustion engine running on a fuel such as petrol or diesel oil.
A piezoelectric actuator is mainly composed, in a known way, of a stack of ceramic elements defining a specified length, which has the property of having its length modified by the action of an electric field, and, conversely, of producing an electric field under the action of a mechanical stress; this stack is placed in an injector between a stop and the valve means, and operates, in summary, in the following manner: when an electrical charge is applied, by means of a voltage, to the piezoelectric actuator, its length increases and opens the valve means of the injector which releases fuel under pressure into the combustion chamber. At rest, that is to say in a closed position of the valve means, there is a clearance between the piezoelectric actuator and the valve means, in order to enable this valve means to be closed and to prevent uncontrolled leaks of fuel towards the combustion chamber.
In order to be stable and have reproducible behaviour, a piezoelectric actuator must be polarized at a reference value, this being done in the factory during the manufacture of said actuator, and before the engine is put into use in a vehicle. This polarization, called the initial polarization, consists in applying an electrical charge by means of a specified voltage, called the polarization voltage, for a period which is also specified, across the terminals of the piezoelectric actuator, thereby causing the crystal structure of the latter to be orientated in the direction of the electric field established in the piezoelectric stack, corresponding to the direction in which the variation of the dimension of the piezoelectric actuator is desired. After the removal of this initial polarization voltage at the terminals of the piezoelectric stack, this stack retains a residual polarization state for its subsequent use.
However, a piezoelectric actuator tends to lose this initial polarization during its use in an internal combustion engine, notably because of the essentially urban use of the vehicle, resulting in low engine speeds and therefore low nominal voltages, considerably below the polarization voltage, for operating the injectors under low fuel pressure. In fact, the nominal supply or operating voltage applied to a piezoelectric actuator to open an injector is adjusted as a function of the requested torque and the engine speed. In particular, it is adjusted, where appropriate, as a function of the fuel pressure which is opposed to the opening of the valve means of the injector, and more generally to the energy required to open the valve means of the injector.
It should be noted that injectors, notably for internal combustion engines running on diesel fuel under high pressure, are preferably designed so that the fuel pressure is used in such a way that it is applied, in the closed position of the injector, from the side of the valve means which keeps the latter in a position in which it bears on its seat.
Other conditions of use, such as repeated cycles of increase and/or decrease in the engine temperature, or long periods without the use of the piezoelectric actuator, corresponding to prolonged periods of immobilization of the vehicle, may also lead to a change in the polarization of the piezoelectric actuator over time.
The depolarization, or drift, causes a contraction of the stack of ceramic elements of the piezoelectric actuator, and a consequent increase in the clearance between the piezoelectric actuator and the valve means. The increase in this clearance results in less precise control of the actuator, which may even lead to the loss of one or more injections of small amounts of fuel, for example what are known as pilot injections, since the injector no longer has time to compensate for the clearance and open the valve for brief time intervals, causing increased engine noise (the main injection combustion takes place with a high pressure gradient owing to the lack of a pilot injection) and pollution, as well as making driving disagreeable. For longer periods of opening of the injector, the drift of the piezoelectric actuator leads to poor control of the amount of fuel actually injected into the combustion chamber.
Furthermore, it is no longer possible to perform a new initial polarization on a previously polarized piezoelectric actuator, since the actuator undergoes processing after the initial polarization in which it is covered with an insulating protective coating and mounted in the injector housing. This it is no longer possible to perform this initial polarization procedure which, in most cases, requires the application of stresses to the stack of piezoelectric ceramic layers and a high temperature of more than 100° C.
However, there are methods of compensating for the depolarization of a piezoelectric actuator with the aim of polarizing the actuator again after the initial polarization applied when the product left the factory, these methods being more or less effective and costly, and generally requiring the dismounting of the injector. In many cases, the piezoelectric actuator, or even the whole injector, is replaced when the depolarization is too great and can no longer be compensated effectively.
The present applicant has proposed, in French patent application FR 1254719 of 23 May 2012, a control method for a piezoelectric actuator of a fuel injector of an internal combustion engine of a vehicle, comprising a step of applying a polarization charge to the actuator during the use of the engine and during fuel injection.
The applicant has continued his research and now proposes a repolarization method which can be used on a one-off basis with the engine stopped, with a minimum of intervention on the vehicle, and which is intended to overcome the aforementioned drawbacks, to some extent at least, and to rapidly improve the performance of a piezoelectrically operated injector which has become depolarized.